Determining object properties with respect to particular optical measurement

ABSTRACT

A method of identifying a surface point or region of an object to be measured by means of an optical sensor providing defined measuring conditions regarding emission of measuring light and reception of reflected measuring light in a defined spatial relationship. The method comprises defining a point or region of interest of the object, determining an optical property of the defined point or of the defined region and deriving an object information base on the optical property. The determination of the optical property is performed by optically pre-measuring the point or region using the optical sensor by illuminating the point or the region with the measuring light, capturing at least one image by means of the optical sensor of at least one illumination (Lr,Li) at the object and analyzing respective illuminations (Lr,Li) regarding position or appearance plausibility with respect to the measuring conditions of the optical sensor.

FIELD OF THE INVENTION

The present invention generally pertains to a method and device forcharacterising a surface of an object, in particular regardingreflection properties of the surface, with view to particulartriangulation measurement of the surface.

BACKGROUND

It is common practice to inspect work pieces subsequent to productione.g. on a coordinate positioning apparatus, such as a coordinatemeasuring machine (CMM), in order to check for correctness of predefinedobject parameters, like dimensions and shape of the object. Moreover, adetection of a surface of an unknown object is of interest in manyindustrial applications. Such measurement typically also may be providedusing a coordinate measuring machine or any other suitable type ofscanning device.

In a conventional 3-D coordinate measurement machine, a probe head issupported for movement along three mutually perpendicular axes (indirections X, Y and Z). Thereby, the probe head can be guided to anyarbitrary point in space of a measuring volume of the coordinatemeasuring machine and the object is measurable with a measurement sensor(probing unit) carried by the probe head. Such probing unit can bedesigned as a tactile probe or an optical sensor providing measurementsof surfaces e.g. based on the principle of triangulation.

In a simple form of the machine a suitable transducer mounted parallelto each axis is able to determine the position of the probe headrelative to a base of the machine and, therefore, to determine thecoordinates of measurement points on the object being illuminated by thesensor. For providing movability of the probe head a typical coordinatemeasuring machine may comprise a frame structure on which the probe headis arranged and driving means for moving frame components of the framestructure relative to each other.

An advantage of using an optical sensor is that it is not in contactwith the part and therefore does not deform it during the measurement ordamage it, as may be the case with a tactile probe.

An advantage of using a line triangulation device in combination with aCMM for measuring a surface is the amount of distance information beingreceived by one time step, i.e. distance values along the entireprojected triangulation line can be determined and respectivecoordinates can be derived. Thus, by moving the sensor along a desiredmeasuring path an object to be measured can entirely be scannedsignificantly faster.

Over the past 20 years, manually operated portable CMM systems (e.g.articulated arm CMMs), comprising typically four segments linkedtogether with one or two rotation axes per linkage and a total of six orseven axes, have become popular for non repetitive measurement tasks onthe shop floor. Line triangulation device are also used on such portableCMMs to greatly increase data capture speed.

Other portable measurement devices where triangulation units are usedinclude optically tracked systems, either using multiple cameras totrack the probe location and orientation or interferometric distancetracking devices, where the rotational axes of the probe are trackedusing an additional camera.

Other applications for line triangulation sensors include fixedinstallations where an object is placed in front of the sensor orsensors and single line measurement(s) of the static object are madesuch that key features of the part can be captured in a single stepwithout the need for expensive positioning systems.

Furthermore, a device for providing a topographic measurement of asurface can be embodied as a (hand-held) device comprising atriangulation sensor, wherein the device is guided along the surface tobe measured—either manually or by a robot—and distance data is acquiredby the sensor while moving the device. Additionally, the position and/ororientation of such device may continuously be determined (e.g. tracked)in a global coordinate system thus enabling a determination of absolutecoordinates corresponding to the object's surface.

In general, triangulation provides a method for scanning a surface infast and precise manner. Measuring devices working on that principle arefor instance known from DE 10 2004 026 090 A1 or WO 2011/000435 A1.

In particular, a line generated by a laser unit, e.g. by moving a laserpoint along such line or by providing a laser fan, is generated on anobject to be measured and the light reflected from the surface isdetected by a camera consisting of a light sensitive image sensor (lightdetector) and electronics to control the image sensor and read out theimage. An image of the reflected light is captured and distanceinformation according to the contour of the detected line is derived.Based thereon, topography of the object's surface can be determined.

For triangulation measurements with high precision, an illumination anddetection of respectively reflected light has to be provided whichcomprises a required level of illumination and an adequate detection ofthe light information. For adjusting illumination so that the reflectedlight reaches the detector meeting its respective detection properties(e.g. signal-to-noise level and saturation limit) WO 2011/000435 A1discloses an approach of an in-advanced illumination in order todetermine a suitable illumination level for the measuring light. WO2007/125081 A1 discloses a further approach for actively controlling thepower of illuminating light in dependency upon an intensity detected bya camera.

However, in case of regions to be illuminated which significantly differregarding their reflecting properties there still remains the problem ofproviding a usable signal over the whole width of a projected laserline. Particularly, surfaces with low roughness, i.e. mirror-likesurfaces such as chrome, are difficult to measure due to stronginhomogenity of the reflected light toward the image sensor. In suchcases, a suitable illumination still will reach its limits and,therefore, the precision of derived distance data would be further below.

Therefore, there remains a problem in identifying regions on the objectwhich provide good reflectance with view to projected measuring lightand other regions which may probably be difficult to measure e.g. due todouble reflections.

SUMMARY

It is therefore an object of the present invention to provide animproved method and a respective triangulation measuring device forenabling a determination of measuring conditions at an object to bemeasured.

Another object of the present invention is to provide a correspondingmethod for detecting double reflections at the object.

Fringe projection is a triangulation technique where a large number of3D points are acquired in a short time by projecting a sequence ofarea-covering patterns while the projector and camera are stationaryrelative to the work piece. Typically, the patterns are “stripes”perpendicular to the baseline direction, and for each direction thesequence defines a unique binary code so that the illumination directioncan be decoded by analyzing the temporal signature on each individualcamera pixel. Typically, the binary code is complemented by a sinusoidal“phase-shift” code which shifts relatively fine stripes by smallincrements to enable sub-pixel precise encoding without having toproject patterns with too high spatial frequencies. FIG. 1 shows anexample of such a phase-shifted code, wherein the pattern sequences andtheir numbers are shown over the projector pixel columns.

One other known pattern sequence consists of two high-frequencysinusoidal patterns with a small difference in spatial frequency whichare both phase shifted. This pattern is less affected by diffuse or“semi-shiny” inter-reflections since the contrast of the high-frequencystripes is reduced significantly after reflection.

A fringe projection sensor has at least one projector and at least onecamera. Some sensors have two cameras and one projector since thisallows high accuracy triangulation between the two cameras even if theprojector is unstable (e.g. due to self-heating), has high lensdistortion etc. With a high quality (=stable) projector, the mentioned2+1 setup also may increase coverage around edges and on shiny surfacesby performing triangulation between all three pairs of devices.

In case of projection of one line, there is for each light pixel in onecamera or projector a corresponding “epipolar line” in a second camera.The epipolar line is the image of the projection defined by a pixellocation, and when searching for a match in the second camera one canthus limit the search to this line. Since one degree of freedom is knowndirectly from this relationship it is only necessary to encode theprojected pattern “along the baseline”. In the other direction, thepattern can be constant “stripes”.

Since the codes for different stripes may not be “orthogonal” it couldbe difficult to extract useful information in cases where reflections onthe work piece cause superposition of several codes. E.g. one projectorpixel column may emit the intensity code 010101 and another 011001. Ifthese are superposed with equal weights the detected code would be021102 and it would not be possible to rule out e.g. the incorrectsuperposition 011101+010001. In an even worse case, the superposed codemay have no variation in intensity (e.g. 100+010=110) and it will thusnot even be possible to detect that there is a reflection. Luckily, therisk of this happening can be diminishing with code length, and thuscould be limited to 10% for an 8-bit code.

Two other types of triangulation sensors are laser line triangulationsensors and laser point triangulation sensors. Both of these havefundamental advantages over fringe projection when it comes to handlingof inter-reflections since only one line or even one point isilluminated at once. In the first case, reflections typically becomespatially separated from the primary signal and can be removed by(non-trivial) image processing. In the second case, most secondaryreflections are not even in the field of view of the single-line camera,and are thus automatically disregarded.

Although a reflection within an epipolar plane is possible, it is muchmore likely that the reflected light comes from some other plane. Todetect such reflections one can perform a second scan with the fringesextending along the baseline instead of across. If the detected code isnot as expected for a direct reflection, one can decide either todisregard this point or e.g. to only perform camera-to-cameratriangulation.

Generally on a very shiny work piece with a complex shape there may beany number of inter-reflections at the surfaces.

However, there are at least two typical cases of occurring reflections:

-   1. The projected pattern is reflected by a primary reflection from a    shiny surface onto a secondary diffuse surface.-   2. The camera sees a mirror image of primary diffuse surface through    a secondary mirror-like reflecting surface.

Where “primary” means the surface first struck by the light of theprojector, secondary the second surface etc.

In the first case it is still possible to perform triangulation(photogrammetry) between the two cameras since at this point of the workpiece they will see exactly the same (overlaid) patterns like a textureon the surface. Triangulation between camera and projector will howevernot work well since multiple projection directions are superposed.

In the second case, not even camera-to-camera triangulation is possiblesince each camera sees a different mirror image.

Today such situation would be avoided by covering the shiny surface witha removable paint. This causes additional uncertainties due to thethickness of the layer and efforts in the preparation and removal of it.

In both cases, it would be helpful to reduce the extent of the projectpattern so that only one of the surfaces is illuminated at a time. Oneobjective of the present invention is a method for segmenting the sceneand projection pattern so that inter-reflections are avoided.

Thus, another objective of the invention is a combination of projectionpatterns and processing algorithms to detect inter-reflections. This isto support a segmentation and reduce the number of erroneous points.

Yet another object of the invention is to avoid any artificial surfacetreatments and to achieve robust and reliable measurement results.

Above objects are achieved by realising the features of the independentclaims. Features which further develop the invention in an alternativeor advantageous manner are described in the dependent patent claims.

In general, some embodiments of the invention relate to a method ofidentifying a surface point or region of an object to be measured, thepoint or region being of particular measuring properties for opticalmeasurement of the respective point or region by means of an opticalsensor. Preferably, points or regions of ambiguous, undefined orproblematic measuring conditions can be identified by means of themethod. In particular, the optical sensor is designed as atriangulation-based fringe- or pattern projection optical sensor. Theoptical sensor provides defined measuring conditions at least regardingemission of measuring light and reception of reflected measuring lightin a defined spatial relationship. In particular, a type, shape and sizeof a projected pattern (e.g. line or grid) are well known.

The method comprises the steps of defining a point or region of interestat the object, determining a surface property related to an appearanceof the defined point or of at least a part of the defined region withrespect to a particular optical measurement using the optical sensor andderiving an object information about measurability applying the definedmeasuring conditions base on the surface property, the objectinformation representing an information about an expected effect on theparticular optical measurement due to the surface property.

The region of interest typically comprises a number of planes (surfaces)of the object which are oriented to each other with defined tilt.

According to the invention the determination of the optical behaviour ofthe point or region is performed either by optically pre-measuring or byanalysing a digital model or by a combination of both.

Optical pre-measuring of the point or at least a part of the region isperformed by use of the optical sensor.

The point or at least a part of the region is illuminated with themeasuring light emitable by the optical sensor. At least one image iscaptured by means of the optical sensor of at least one illumination atthe object caused by illuminating the object and respectiveilluminations (the at least one illumination) are analysed regardingposition and/or appearance unambiguity (plausibility) with respect tothe measuring conditions of the optical sensor.

In other words, the illumination on side of the object can be analysedregarding its position at the object, its shape, size and possibledistortions or the like. The type of measuring light, i.e. type, shape,dimension of a pattern and/or direction of projection, typically ispre-known. The optical sensor preferably comprises at least one camerahaving a photo-detector for acquiring image information like an image ofa projected pattern.

In particular, by applying a defined illumination, the appearance ofsuperimpositions of illumination patterns at the surface can berecognised. If the captured pattern does not comprise unambiguousconditions (contrast, size, shape etc.) this may be a hint on occurringsuperimpositions or other undesired illumination effects like doublereflections.

On the other hand, an analysis of the digital model of the object to bemeasured is executed by digitally or virtually aligning the digitalmodel in accordance with an in-reality orientation of the objectrelative to the optical sensor according to a given measuringconstellation. Appearance properties of the point or region aredetermined based on the aligned model regarding an illumination with themeasuring light from the optical sensor in the orientation of the objectrelative to the optical sensor.

With such approach surface properties of the object can be determined byuse of a respective algorithm and an expected effect on a plannedmeasurement can be derived based only on such model-analysis. Inparticular, reflection (and roughness) properties of the analysed regionare known, e.g. from a CAD-model.

According to some embodiments of the invention, the opticalpre-measuring comprises determining at least one image-position in theat least one image of respective illuminations at the object, checkingfor positional plausibility of the at least one image-position withrespect to the measuring conditions of the optical sensor, in particularby considering an axis of illumination of the sensor, and generatingposition unambiguity information based on the checked positionalplausibility.

In case the projected pattern (point, line or grid etc.) appears at theobject at a position which would be expected due to a position andorientation of the sensor relative to the object, such appearance isdetected and recognised by applying the method. In such case, forinstance double reflections of a projected pattern can be excluded to asignificant probability.

However, in case an illumination is detected at a position different toan expected position this can be an evidence for the occurrence ofdouble reflections when illuminating the object accordingly and, hence,reduced measuring quality can be expected with an optical measurementsof the object as planned there.

Hence, “unambiguity information” means information about if the patternoccurring at the object appears there in a way as expected and desired,i.e. basically in a way as generated on side of the optical sensor, orif the pattern is different to an expected pattern, e.g. due toreflections between exposed surfaces. Therefore, the unambiguityinformation is kind of a measure of plausibility of appearance of thepattern at the object.

According to some embodiments of the invention, image data of the atleast one illumination can be generated, the image data comprising atleast two pictorial representations of the at least one illumination atthe object from at least two different poses. For each of the pictorialrepresentations the at least one image-position of the respectiveilluminations at the object can be determined and the image-positionscan be checked for consistency regarding the measuring conditions.

In particular, one can check if the image-positions represent a commonillumination based on an illumination direction for the measuring light.

In further embodiments of the invention, a spatial position derived by atriangulation-based determination based on the image-positions, inparticular by means of photogrammetry, is compared with a position of anillumination axis or illumination plane of the measuring light. Inparticular, one can check if the position derived by thetriangulation-based determination lies on the illumination axis or theillumination plane.

For instance, each of the two cameras of an optical sensor provides animage which covers one illumination at the object (as image data). Foreach image an image-position (in the image) for the covered illuminationcan be determined (e.g. with respective image coordinates, e.g. relatedto the planar dimensions of image sensors). Considering a known spatialrelationship of the cameras (and e.g. by rough knowledge about the shapeand size of the object to be measured), one can, e.g. based onphotogrammetric approaches, determine if the position of theillumination at the object is plausible (unambiguous) or if suchillumination would be caused due to ambiguous reflection effects. Inparticular, in course of such determination a position and orientationof an epipolar line (e.g. by considering a respective illumination axisor the illumination plane or a virtual line defined by one of thedetermined image-positions) can also be considered in order to moreefficiently determine if the covered illumination is plausible in lightof given measuring conditions or not.

Concerning the illumination of the object, according to an embodiment ofthe invention, the illumination of the point or region is provided bythe measuring light being in form of a line of light, a light pattern(e.g. a line pattern having at least two lines being transversely, inparticular orthogonally, aligned to each other), or a light spot. Aprojection patter can for example be represented by a laser line grid.

According to some embodiments of the invention, the opticalpre-measuring comprises moving the measuring light over the objectaccording to a defined scanning path, in particular in defined scanningdirection, continuously detecting a position of an illumination causedby the moving measuring light at the object, deriving a movement pathfor the illumination at the object, in particular an object movingdirection, comparing the scanning path to the derived movement path andgenerating position unambiguity information based on the comparison.

Applying such an approach enables to detect a (double) reflectedillumination at the object due to differing moving directions of theprojected and detected illumination. An illumination at the object whiche.g. is mirrored moves in a different direction than the one directlyprojected onto the object.

Ambiguous measuring conditions can also be identified according to theinvention by analysing contrast and/or intensity of the at least onecaptured illumination, comparing the contrast and/or intensity to arespective reference value and generating appearance unambiguity(plausibility) information based on the comparison.

A projected pattern which is directly reflected back and detected by thecameras of the sensor comprises a contrast and/or intensity whichsignificantly differs from contrast and/or intensity of the patternbeing first reflected from a first surface region of the object onto asecond surface region of the object and afterwards detected with thecameras, i.e. the illumination caused at the second surface region iscaptured by the cameras. The contrast and/or intensity may for instancebe reduced due to superimposition of the directly projected pattern andthe reflected pattern at the second surface region.

In particular, the reference value is defined by a contrast level of oneof the illuminations generated or captured at the object. Moreover, thereference value may also be represented or derived by a contrast orintensity level which can be expected with a respective illumination.Preferably, object properties like material or surface finish of theobject are considered here as well.

According to embodiments of the invention the point or region can beilluminated with measuring light defining a fine pattern with successivebright and dark illumination regions.

Regarding the definition of the point or region of interest of theobject according to the invention, a first polygon in a first cameraview and a second polygon in a second camera view of the object can bedefined. The first and the second polygon define (at least partly) acommon region at the object. Topographic information of the commonregion is derived based on photogrammeric processing using the first andthe second camera view. In particular, the topographic information isreferenced with a projector coordinate frame of the optical sensor.

According to some embodiments of the invention, the point or region ofinterest can be defined by use of a coaxial view to the object, whereina viewing axis of the coaxial view basically corresponds to an emissionaxis of the measuring light, in particular wherein the optical sensorcomprises a light emitting unit adapted to provide emission of themeasuring light according to the emission axis and a camera adapted toprovide reception of the reflected measuring light according to theviewing axis, wherein the emission axis of the light emitting unit andthe viewing axis of the camera are coaxially aligned, in particularwherein the camera is represented by a light receiving unit of theoptical sensor.

For instance, an additional camera may be provided the optical axis ofwhich may be coupled to the projector's optical axis for providingcoaxial optical alignment.

With respect to the present invention the optical measuring can beperformed as a pre-scanning process for the point or region of interest.I.e. the method according to the invention can be executed in advance ofa later measurement of the object in order to derive particularmeasurement parameters to be chosen for obtaining reliable measurementresults for a respective point or region at the object.

Concerning the analysis of the digital model, according to an embodimentof the invention, the digital model is segmented into defined pieces ofthe model each of which representing a part of the object, in particularusing a segmentation grid. The model is analysed concerning surfaceproperties of the object, parts of the object with similar or identicalsurface properties are determined and the parts of the object withsimilar or identical surface properties are referenced in respectivepieces of the model.

The result is for instance a grid, wherein in each of the segmentsdefined by the grid a respective part of the object which comprisesdefined surface properties (e.g. reflection behaviour) is identifiedand/or marked. Based on such segmentation, measurements can be planed tobe performed in a way such that suitable measuring conditions are chosenor applied for measurements of the identified areas within respectivesegments. By that more reliable and faster measurement of respectiveparts of the object can be provided.

In particular, the parts of the object with similar or identical surfaceproperties are assigned to a first group, particular measuringproperties are defined for the first group and triangulation measurementof the first group are performed by applying the defined particularmeasuring properties.

Of course, there may be defined more than one group, wherein parts ofthe object assigned to a respective group comprise similar measuringconditions and thus require particular measuring parameters. Inparticular, each group defines different measuring conditions.

As mentioned above, the method according to the invention is apre-determination method for providing suitable measuring parameters forlater detailed measurements with the optical sensor. In other words, themethod provides determination of the surface property of a surfaceregion at the object with respect to defined sensor properties and to adefined alignment of the sensor relative to the object, whereinmeasuring parameters of the sensor (for a successive detailedmeasurement of the surface) are set on basis of such pre-determinedsurface property or conditions. Hence, the derived surface property maybe valid for a specific arrangement of object and sensor only.

The invention also relates to a triangulation-based optical sensor, inparticular fringe- or pattern projection optical sensor, with which amethod of above can be performed. The optical sensor comprises a lightemitting unit with a light source, in particular a laser diode, forproviding defined measuring light, in particular a light pattern, atleast one light receiving unit, e.g. a camera, having a detector, e.g. aCCD-detector, for detecting measuring light reflected and received froman object to be measured and a controlling and processing unit forderiving distance information based on the detected reflection. At leastan arrangement of the light emitting unit and the light detection unitwith known spatial position and orientation relative to each other, inparticular according to the Scheimpflug criterion, defines measuringconditions of the optical sensor. It is to be understood that suchmeasuring conditions may further be defined by specific properties of agenerated light pattern or by a detection sequence of a camera.

According to the invention, the controlling and processing unitcomprises a pre-measuring functionality by execution of which adetermination of an object surface property related to an appearance ofa defined point or of at least a part of a defined region of the objectwith respect to a particular optical measurement using the opticalsensor is performed. The pre-measuring functionality is performed byeither optically pre-measuring the point or region or by analysing adigital model of the object to be measured or by a combination of both.

Optically pre-measuring is performed by illuminating the point or atleast a part of the region with the measuring light, capturing at leastone image by means of the light receiving unit of at least oneillumination at the object caused by illuminating the object andanalysing the at least one illumination regarding position and/orappearance unambiguity (plausibility) with respect to the measuringconditions of the optical sensor. Analysing the digital model isperformed by digitally aligning the digital model in accordance with an(in-reality) orientation of the object relative to the optical sensorand determining appearance properties of the point or region based onthe aligned model regarding an illumination with the measuring light inthe orientation of the object relative to the optical sensor.

In particular, the pre-measuring functionality is adapted to execute amethod as described above or below.

According to some embodiments of the invention, the light emitting unitis embodied as a projector and defines an emission axis, thetriangulation-based optical sensor comprises a camera which defines aviewing axis, in particular wherein the camera is represented by the atleast one light receiving unit. A projector object surface of theprojector and a camera image sensor of the camera are arranged so thatthe emission axis and the viewing axis are coaxially aligned. Inparticular, the projector and the camera are arranged so that theemission axis and the viewing axis are coaxially aligned.

In particular, the triangulation-based optical sensor, in particular thelight emitting unit, comprises an optical assembly, wherein the opticalassembly comprises at least a beam splitter and a lens for providing theemitting and the viewing axes to be coaxially aligned. E.g. the opticalassembly may be designed with one beam splitter and two lenses, a firstof the two lenses providing an adequate image to the camera and a secondof the lenses providing desired projection onto the object. Moreover,the projector object surface and the camera image sensor (in particularthe camera) are integrally arranged within the projector.

Some embodiments further relate to a computer program product havingcomputer-executable instructions implemented for executing andcontrolling at least the step of determination of the surface propertyof method described herein above or below. The determination of thesurface condition comprises optically pre-measuring the point or regionusing the optical sensor and/or analysing a digital model of the objectto be measured of the respective method. In particular, the computerprogram product is implemented on or provided (e.g. provided by a dataserver unit or cloud) to a controlling and processing unit of an opticalsensor.

BRIEF DESCRIPTION OF THE FIGURES

The method according to the invention is described or explained in moredetail below, purely by way of example, with reference to workingexamples shown schematically in the drawings. Specifically,

FIG. 1 shows a pattern of phase-shifted code, wherein the patternsequences and their numbers are shown over the projector pixel columns;

FIG. 2 shows the effect of a reflection caused by illuminating a shinysurface of an object;

FIG. 3 shows another example for occurring reflections on a particularsurface;

FIG. 4 shows a further problematic occurrence of reflections caused byillumination by means of a projector of an optical sensor;

FIG. 5 shows a principle of detecting reflections by means of definedmovement of a projection at the object;

FIG. 6a-b show vector diagrams related to reflection identification byscanning;

FIG. 7a-b show the effect and handling of patterns caused by shifted ormirrored projections;

FIG. 8 shows a directly projected pattern represented by black soliddots and a respectively reflected pattern represented by striped dotswith defined period;

FIG. 9a-b show a homogenous fringe pattern which is illuminated on theobject and a superposition with a respective reflection; and

FIG. 10 shows a model-based principle of testing for inter-reflectionbetween surfaces;

FIG. 11a-b show embodiments of optical systems according to theinvention providing coaxial fields of view of a camera and a projectorof a triangulation sensor;

FIG. 12 shows an approach according to the invention of determiningambiguous reflections at an object by projecting orthogonal linepatterns; and

FIG. 13 shows a method according to the invention of how to fuse regionswith similar reflection behaviours.

DETAILED DESCRIPTION

FIG. 2 shows the effect of a reflection caused by illuminating a shinysurface 14 of an object, for simplification with a single point.

An incident spot L_(i) projected by a 2D projector 13 of a triangulationsensor 10 on shiny tilted surface 14 causes a double reflex L_(r) on asecond matt surface 15 (i.e. the region of interest comprises at leastparts of both surfaces 14,15 which can be captured by the cameras). As aconsequence the determination of the point directly illuminated by thespot on the object is no longer unambiguous due to the second reflexL_(r). It is also likely, that the second reflex L_(r) appears brighterdue to a stronger scattering of the matt surface 15. Without any furtheranalysis this setup would cause an outlier in the measurements or even alarger region of the shiny 14 and matt surface 15 will not bemeasurable.

The projection direction passing point L_(i) corresponds to an epipolarline in the image plane of the cameras 11,12.

Along this line the location of the projection is determined in 3 Dcoordinates.

Camera 11 will identify L_(A) as the virtual location of the reflexlocation L_(r) and for camera 12 this will be the virtual locationL_(B). The inconsistency of the two locations L_(A) and L_(B) is adirect indication of a misleading due to the double reflex. Such doublereflex represents a property of the respective surfaces, in particularsurface 14. The region (both surfaces) can be defined as a doublereflecting region.

According to the invention such an inconsistency is checked based on theknowledge of possible positions of the projected spot due to a givenprojection direction and based on the given relative position andorientation of the cameras 11,12 and the projector 13. Respective imagescaptured by the cameras 11,12 are compared to each other, whereinimage-positions of the two locations L_(A) and L_(B) are determined inthe images. Considering the camera orientations and the projection axisthe result here would be that the locations L_(A) and L_(B) do notrepresent one single spot at the object but would have to be assigned totwo different positions at the object. As there is only one single spotprojected such result gives information about occurrence of ambiguousmeasuring conditions there.

As a consequence a planned measurement of the illuminated position canbe adjusted based on the identification of such ambiguity. E.g. apattern to be projected of an angel of incidence may be adjusted toprevent a significant or dominant double reflex.

In FIG. 3, the point to be measured is L_(D) which is directlyilluminated by the ray ID. Superposed with the directly reflected light,the cameras also see light from the I_(R) ray which is reflected fromthe shiny surface 14 at location L_(R). Depending on the surfaceproperties of the two surfaces 14,15 (=region of interest), either thedirect or the reflected light may be stronger. For an ideal mattesurface the intensity relationship should be the same seen from bothcameras 11,12, but for “semi-matte” surfaces this may not be exactly thecase. If the reflected light is decoded and triangulation performedagainst the projector 13, camera 11 will think that it comes fromlocation L_(RA) and camera 12 will think that it comes from L_(RB),which is an invalid situation since the projector cannot illuminate bothof these points at the same time (a surface at L_(RB) would shadowL_(RA)).

However, when performing triangulation between the two cameras 11 and12, the correct location L_(D) will be found since both cameras 11,12see essentially the same pattern.

Again, uncertainty in defining a correct and unambiguous object locationrelated to the projected spot can be found by comparing respectivelyidentified image-positions of the spot in a first image captured bycamera 11 and in a second image captured by camera 12. In particular,knowledge of the orientation of the projected laser beam and/or theorientation and position of a virtual epipolar line is considered withthat process.

FIG. 4 shows a further problematic occurrence of reflections caused byshiny surfaces. It is intended that point L_(D) is observed by the twocameras 11 and 12. Due to the reflection of the shiny surface 14 theobservation is mirrored to the matt surface 15. In this way the image atpoint L_(D) is occurring from point L_(R1) for camera 12 and from L_(R2)from camera 11. At these locations the projector will illuminatedifferent patterns which will cause a complete misinterpretation in theanalysis of the two camera images. The analysis of camera 11 will yieldto a location L_(RA) (crossing of the epipolar line of the projectorwith the viewing direction of the camera) and for camera 12 to alocation L_(RB). From this discrepancy one can conclude an appearanceproblem of the observed surface.

Hence, here ambiguity is given by multiple reflections of the initiallygenerated spot L_(D). According to the invention such ambiguity can bedissolved by image processing and comparing respective image-positions.

Each of above examples shows particular difficulties in measuringrespective objects which provide such or similar surface conditions, inparticular in combination with a respective orientation of the objectrelative to a measuring sensor, e.g. to a triangulation sensor. In thefollowing, approaches (as partly already outlined above) of identifyingproblematic regions at an object to be measured according to theinvention are described in more detail.

Triangulation with a single point of illumination is the most robustapproach to detect reflections but also the slowest. Thanks to area scancameras in a fringe projection sensor it is also in many cases possibleto see where the secondary reflections occur. A quick low resolutionpre-scan over the object with a single projected point observed by twocameras will show directly where problematic surfaces are that causedouble reflexes due to inconsistency of the reflex-positions between thecameras as described above. Depending on the complexity of the objectseveral points might be projected simultaneously onto the object toreduce scanning time.

To further increase speed while still being robust on shiny surface morethan full area fringe-projection one could perform the pre-scan using acontinuous line instead of a point, thus capturing e.g. 1000× as muchdata per image frame. In the acquired images one will see both theprimary line as well as reflections of the same. By using methods knownfrom laser line triangulation sensors it is in many cases possible todetermine which line is the primary one and for instance thus generate a3D model of the object.

Especially when using two cameras it is easier to detect doublereflection since only points on the primarily illuminated plane areconsistent when triangulating each camera against the projector. Thisapproach will not work for double reflexes appearing within theilluminated plane (along the projection line). A second perpendicularscan can be performed to remove this uncertainty.

Unlike for point projection, it is however not as easy to determine fromwhich primary point each reflected point originates, so segmentation(identification or definition of particular regions or zones) based oninformation from a line projection pre-scan is more difficult. Just asfor point projection, in some cases it may be possible to increase scanspeed by projecting multiple lines at once.

Because double reflections appear only on secondary surfaces that aresomehow tilted to the first surface, the movement of the projectionpattern (either a point, line, or fringes) will appear on the secondsurface in a direction in which the scanning path on the first surfaceswill cross the extrapolated tilted second surface. Thus, by detecting amovement of a reflection at the object and comparing a movementdirection to a direction of scanning the laser line or spot relative tothe object one could determine if the detected reflection is a primaryor a secondary reflection.

Above approach is also shown with FIG. 5. By shifting the projectiondirection (scanning) from point L_(i1) to L_(i2) along the scanningvector direction V_(i) the reflection moves from point L_(r1) to L_(r2)along the reflex vector direction V_(r) which has a vector componentthat is perpendicular to V_(i).

FIGS. 6a and 6b show vector diagrams related to such identification byscanning. Scanning on a flat object perpendicular to the optical axis ofthe projector and observed by a camera defines the x-axis. Now in casethe object is tilted the scanning path observed by the camera will nolonger be along the x-axis but will have a component in y-direction.This is shown for the first surface by the vector V_(i) alongside thepoint moves from L_(i1) towards L_(i2) (assuming the surface is planebetween these two points). In parallel, the reflex location on thesecond surface will move from L_(r1) to L_(r2). For visualization thestarting locations of L_(i1) and L_(r1) are shown placed at the originof the coordinate system in FIG. 6b . Knowledge about such behaviouralso enables to distinguish an initially desired illumination from a(secondary) reflection of such projection.

The vector V_(r) has a component along the x-axis that is oppositetowards V_(i). It will be always on the left side of the coordinatesystem defined by the scanning direction of the primary point.

This opposite behaviour and form of incident and reflected pattern isalso represented by the orientation of the pattern in respectivelycaptured images. Due to the mirroring, movement in the projected pattern(phase shift) will change direction after reflection so that the axes ofthe projected pattern (projector pixel axes) in the reflection will berotated and/or mirrored. Such effect is shown in context of FIGS. 7a and7 b.

FIG. 7a shows an illumination of an object with a pattern represented bytwo arrows 21 i (projector x-axis) and 22 i (projector y-axis) generatedand emitted from a projection centre 20 of a respective projector ase.g. shown as triangulation sensor 10 in FIG. 2. The initially projectedpattern is mirrored at a shiny surface 24 of the object. As aconsequence a respectively mirrored pattern represented by the mirrored(reflected) arrows 21 _(r) and 22 _(r) can be imaged at the surface 24.

FIG. 7b shows an object being illuminated with a pattern againrepresented by the arrows 21 _(i) and 22 _(i). The illuminated surface24 of the object is of comparatively high reflectivity. The projectedpattern is thus reflected and a reflection (arrows 21 _(r) and 22 _(r))of that pattern is generated on a second (e.g. matte) surface 25 of theobject. Such reflection comprises a component with opposite direction ofthe pattern in x-direction.

In particular, a captured image may be rectified against the projector.The acquired images thus may be transformed such that their pixel rowsare aligned with the projector pixel rows, and the horizontal (=alongbaseline) projector pixel axis is thus also horizontal in the images.The vertical projector axis may be rotated due to an object surfaceslope, but will at least not change sign. Then, any other motion vectorscan be indications of double reflections.

To probe the projector pixel axes one can project a pattern shifted toat least three positions: one to define the origin and two with a smallshift in two non-parallel directions. Typically, horizontal and verticalshifts may be chosen. The pattern further can have structure in both thehorizontal and vertical direction to allow correct motion estimation.The images can then be analyzed using algorithm for 2D motion estimatione.g. optical flow or phase-based motion estimation. Since the motionwould only be analysed locally it is not required that the pattern isnon-repetitive, thus a regular grid of dots or lines or a random dotpattern will suffice.

Instead of a 2D pattern and three images, it is also possible to projectonly a 1D pattern (e.g. fringe, stripe) but then use four images sincethe same origin-image cannot be used for both directions. The imageanalysis will in that case be different since the out-of-axis componentswill then be measured from the fringe direction in single images whilethe in-axis components are computed from the motion vectors between thetwo images.

In the end, the reflected pattern can be superposed with the directpattern, and there may thus be multiple motion directions in a singleneighbourhood. To be able to distinguish both motions, it is beneficialto use a kind of sparse pattern consisting e.g. of single bright pixeldots separated by three dark pixels so that the dots are clearlyseparated at least for some offset (FIG. 8). Multiple shifts (instead ofjust two as discussed above) will also help identify the two sets ofdots and corresponding motions. The total shift could be e.g. one periodin step of one pixel, so in total seven patterns to probe both projectorpixel axes.

With FIG. 8 a direct pattern (directly projected) is represented byblack solid dots and a respectively reflected pattern is represented bystriped dots with a period equal of four spot widths.

The pattern could be coarse enough that features are not too blurredafter reflection. At the same time, in cases where the ordinary fringepattern gets totally blurred the reflection would no longer be a bigproblem. In the end, the projector axis probing pattern can have aperiod similar to that of the fringes in the regular pattern sequence,at least in case of a two-frequency pattern.

Alternatively or in addition, contrast and/or intensity distribution inan image can be analysed in order to identify direct and secondaryilluminations at the object.

In a first illumination of the object with a fine pattern secondaryreflections from shiny surfaces can be superimposed on the directpattern on affected areas. The second reflection will be likely rotatedto the first illumination. This can cause a quite strong reduction ofthe visibility and contrast of the pattern.

FIG. 9a shows a homogenous fringe pattern 30 which is projected onto theobject (without double reflections).

As can be seen in FIG. 9b , due to the reflection of the shiny surface14 it comes to a double exposure on the matte neighbouring surface 15.This can have a significant impact on the observed pattern. The fringecontrast can be strongly reduced.

Also it may occur, that the reflection from the shiny surface 14 will bemore blurry because typically also shiny surfaces 14 have a residualroughness scattering the incident light.

Hence, by projecting a sequence of binary fringe patterns 30 andanalyzing the contrast sequence for each pixel one can conclude whichpixels are affected by double reflections. Normally, if there is onlythe direct incidence of a fringe pattern one can expect two intensityvalues for the bright stripes and the dark stripes. A further indirectreflex from a shiny surface will add another two intensity values thatyield in-total a new mixed intensity distribution, that is much broaderand less pronounced.

By extending the analysis to small regions instead of single pixels onecan further improve the sensitivity since the risk that several pixelsshow false negative results is small.

By analysis of the intensity distribution 30 over the object in smallareas the impact of a second, indirect illumination becomes visible.

A further aspect of the invention relates to the use of a digital (CAD)model. In case a digital model of the object is available the object canbe pre-scanned to identify the orientation (alignment) relative to themeasurement system (triangulation sensor), and all reflex conditions canbe identified if the surface characteristics are known (e.g. roughness,reflectivity of the projected wavelength). However, in reality theseestimations are changing due to changing conditions of the test objectover manufacturing processes.

The object can be split into surface regions of similar inclinationangles (e.g. basically relating to the same surface normal) and thisinformation can be used later on for adaptive illuminations in course ofthe measuring process.

The alignment of the digital model in accordance with the object can bedone by several methods, e.g.:

-   -   pre-scan with a line or a rough pattern,    -   matching 2D features (edges, corners, bore-holes) by        photogrammetry or    -   manually by the user (rotation of the digital model).

Using a rough 3D model of the object, either obtained by a pre-scan orfrom a CAD model, the purpose of a segmentation is to divide theprojection pattern into a number of segments which do not create doublereflections within each segment. As mentioned above, one could e.g.split the object into surface regions of similar inclination angle sincesuch surfaces cannot interfere over a single reflection.

With FIG. 10 the principle of an alternative option to actively test forinter-reflection between surfaces is shown. The model 40 of the objectcan be sliced by a regular grid in projector pixel coordinates. Then,for each grid cell one can further separate non-connected surfaces asshown in the marked row 41 of cells where the striped areas would beseparated from the rest of each cell. After this pre-segmentation, eachpre-segment would in turn be illuminated by the projector and respectiveimages are analysed to see which other segments are affected by doublereflections. As for instance only one binary pattern per segment isprojected and the image quality does not need to be perfect, this can bedone quite fast (e.g. as fast as the camera allows). E.g. 200pre-segment images could be projected in one second using a 200 framesper second camera.

After the analysis of which pre-segments interfere, a smaller number oflarger segments can be formed and can then be measured using the fullfringe projection sequence. Each pattern in the sequence can then bemasked to only illuminate the segment of interest, and only the areacorresponding to the segment as seen by each camera may be analysed.

An alternative or additional non-automated method according to theinvention is based on the selection by the user to identify criticalareas that can cause double-reflections on other surfaces, either insidethe CAD model or based on data available after a pre-scan of the object.If a CAD model is available, the selection could be based on the CADgeometry and done in 3D, otherwise the user could e.g. define thesegments by drawing polygons onto a camera image, which would then betransformed to projector space by mathematical projection onto the rough3D model.

Even without a rough 3D model, the user can manually select segments bydrawing polygons, preferably in the images of both cameras so that the3D shape of the polygon is known. It can then trivially be transformedto projector space.

Alternatively or additionally, to avoid having to select areas in twoimages, one approach is related to add a camera which is coaxial withthe projector (optical axis of the camera is coaxial to the projectionaxis of the projector). Since this camera sees the scene from the samepoint as the projector projects, there is a fixed 2D-to-2D relationshipbetween the respective camera image and the projected pattern. Hence,one could easily transform the selected area (in the camera image) toprojector space without any 3D model. In such an image one could alsoperform segmentation based on 2D image features such as edges. Inparticular, alternatively to a coaxial alignment, it may be sufficientto place a small camera as close as possible to the projector.

A further option to avoid both the double selection and a further camerais to actively find each node point in the model polygon by iterativelyadjusting the position of a projected single dot until the dot as seenby the camera is in the selected location. It can be only necessary tosearch in one degree of freedom thanks to the epipolar condition. Foreach user click on the camera image, the sensor can thus quickly scanthe corresponding epipolar line to find the right position. This scancould either be done using a binary pattern (like the fringe projectionit-self), by moving a single dot or iteratively reducing the size of asingle line segment.

Yet another option is to let the user define the polygon directly inprojector coordinates. To directly see where the node would end up fromthe view of each camera, the mouse pointer and/or the polygon so far canbe projected onto the scene using the projector and then imaged liveusing the camera instead of showing it directly on screen.

By registering the shape of the polygon in the camera images, thesoftware will also know which image areas to analyse when performing themeasurement. In case of very strong reflections it may be necessary toin sequence project single points to the nodes of the polygon instead ofthe whole polygon at once.

Concerning an adaptive illumination to form the respective patterns(e.g. the striped segments) required for the methods above, aprogrammable pattern generator such as a DLP or LCD array can be used onside of the projection unit. Typically, such component can generate botha segment mask and a pattern or (fringe) pattern sequence. Fixed slidescan also be used for generation of the pattern (e.g. in order togenerate more accurate or higher frequency sinusoid patterns), wherein aDLP or LCD can be used only to define the masking area.

To further improve the robustness another (or more) projector can beadded. One benefit of that is that it will be easier to avoid specularreflections. Often on shiny surfaces one of the cameras is blinded byspecular reflections. If there is at the same time a double reflectionwhich makes camera-projector triangulation unreliable it is difficult toacquire data. By having a second projector more points will be visiblewith good exposure and contrast in both cameras at the same time.

Instead of (or additionally to) figuring out the segmentation based ongeometrical data or mapping of the double reflections, one could alsomeasure difficult surfaces iteratively. Starting with illumination ofthe full area, the area can be iteratively reduced by excluding pointsas soon as they are captured with high enough confidence. Such processmay be performed with the following steps:

-   1. Perform fringe projection measurement of remaining area (at    start: full area);-   2. Extract 3D points where measurement quality is good (no double    reflections, proper exposure etc.);-   3. Remove the corresponding pixels from the illuminated area for the    next iteration;-   4. Run another iteration (from step 1), repeat until all points are    captured or maximum number of iterations reached.

By using an LCD or DLP projection method not only the projection patterncan be chosen flexible but also the areas to be illuminated. The problemof the double reflexes is the super-position of the direct pattern andthe reflected one, what can cause severe errors in the computation ofthe 3D coordinates resulting in outliers or unmeasurable areas.

According to an embodiment of the invention segmentation or patch-fusioncan be performed as follows. If having N patches or regions (e.g. in agrid) there are N×N combinations of source and target patches. All ofthese combinations can be analysed by projecting the N patterns whiletaking N images. Then, the goal is to by calculation (no newmeasurements) divide the patches into a minimal group of larger segmentswithout internal crosstalk. One way to fuse the patches or regions is tostart with a patch (the first one, randomly selected etc.) and patch bypatch add more from the neighbouring ones until no more neighbouringcross-talk-free patches exists. Then, the patch fusion process isrepeated starting at another unallocated patch. After the grouping ofpatches into segments, the segments can be analysed in the same way tocombine sets of non-connected segments into even larger groups tofurther reduce the measurement time.

When fusing patches, the brightness of the patch can also be taken intoaccount so that only patches with a similar brightness are in the samesegment. Then, the exposure time can be optimised for each segment tolimit the required camera dynamic range.

After dividing the projection image into segments as described above,each can be measured using standard fringe projection methods. For eachof the segments, an additional quality check can also be done (asdescribed above).

By one of the previously described methods to identify the criticalareas that can cause reflections on neighbouring areas, these areas canbe measured (illuminated) step by step in a further procedure:

-   1. First all areas are illuminated, wherein the dynamic of the    system (defined by e.g. the sensitivity of the camera sensor,    exposure time, aperture of the camera lens and brightness of the    projector) has to be large enough so that the shiny surfaces are    measurable. Areas that suffer from double reflexes can be ignored in    the computation of the point cloud data in that step.-   2. In a second step, only the areas that show double reflexes are    illuminated and evaluated, i.e. respective point clouds are derived.-   3. Afterwards both point cloud results are combined to one.

According to an embodiment of the invention a camera may be located sothat the optical axis of the camera is coaxial with a projection axis ofthe projector. By that a parallax-free perspective can be provided.

The method to identify and taking care of surfaces with an appearancethat shows ambiguity can be done either by cameras looking on the scenefrom an off-axis perspective or from an on-axis camera, that shows aparallax-free perspective. In case of an on-axis camera location theanalysis of problematic surface can be easier done and more direct. Arespective implementation can be provided by an additional camera and anoptical setup to overlay the on-axis camera with the projectiondirection.

In order to make the evaluation of pre-scan data less complex, fasterand more accurate it could be beneficial to have one camera which sharesthe field of view of the projector. With its nodal point at the same(virtual) location as the projector, there will be no parallax betweenthe two and thus a one-to-one correspondence between camera 2D imagecoordinates and projector coordinates. Thus, no 3D-reconstruction orknowledge of a CAD model would be necessary to interpret the data sincefor each projected pixel it is known at which camera pixel a directreflection of this light will be imaged, regardless of the shape of theobject. In a preferred embodiment, such an on-axis camera that could bepart of the projector would be only used to detect appearance ambiguityand not be used for triangulation measurement purposes.

In FIG. 11a such a setup is exemplarily illustrated where the same lens51 is used for both a camera image sensor 52 and a projector objectsurface 53 of a triangulation-based fringe- or pattern projectionoptical sensor. In case the projector object surface (e.g. DLP or liquidcrystal array) is not the same size as the camera image sensor, an extraadaptation lens (not shown) may be added e.g. in front of the imagesensor 52 so the camera has at least the same field as the projector.The fields of view are combined by a beam splitter 54. In particular, insuch embodiment the camera sensor 52 is part of the projector 13′. Inother words, camera and projector may be integrally formed.

According to an alternative setup of FIG. 11b a beam splitter 54 is putin front of the projection lens 51 a and there is a separate lens 51 bfor the camera. This camera lens 51 b then would be adjusted so that itsnodal point 55 b is at the same or close to the same distance D from thebeam splitter 54, i.e. at the same virtual location as the nodal point55 a of the projection lens 51 a (parallax free observation). Inparticular, in such embodiment the camera sensor 52 is part of theprojector 13″. In other words, camera and projector may be integrallyformed.

In general, according to respective embodiments of the invention, anumber of patterns can be projected onto a scene to characterize thereflections within the object. Thanks to a coaxial camera setup, it isbeforehand known which pixels of the camera are lit by the primaryreflection. Any detected light in other pixels is thus due tointerreflections or “cross talk”. Using this information regarding thecross-talk between different areas of the projection space an optimalsegmentation (defining regions with ambiguous reflections and regionswithout such ambiguity) of the scene can then be constructed.

The most reliable way to perform a scan would typically be to illuminateonly one projector pixel at a time. This would however comparativelytime consuming since a typical projector image consists of millions ofpixels and the frame-rate of cameras used is typically not more than afew hundred images per second.

To speed up the measurement, one can illuminate sets of multiple pixelsin the same illumination. By doing this, there is a risk that there areundetected reflections within such a pattern. Thus, a method to detectsuch internal reflections is proposed. After having determined which ofthe patterns that may have interreflections, one can then proceed withdividing them into multiple smaller sub-patterns with less risk ofinterreflection.

For instance, one could project long thin stripes at varying angles. Foreach stripe, the reflection may be a semi-continuous thin distortedstripe at some offset from the primary line. It is then not known whichpart of the illuminated stripe is the source for each part of thereflected line. By performing another scan with stripes at anotherangle, this information can be deduced. This is illustrated in FIG. 12.A determination of a source point 61 a and destination point 61 b ofreflection by projection of orthogonal lines 62 a and 63 a is shown. Thecaused reflected orthogonal lines 62 b and 63 b are also shown.

For instance, one could also divide the projection image captured onside of the camera into larger patches or regions according to a grid.To help detect interreflections within each patch, the neighbouringpixels can be analyzed. If they show signs of cross-talk, there is alsorisk of an internal crosstalk, and the patch is divided into smallersub-patches which are tested in the same way. Another way to detectinternal cross-talk is to project a pattern with a finer structure (e.g.checkerboard pattern, a grid etc.) within the patch and check at thedark parts that there is no internal cross-talk.

One could also perform a first scan using a single lit pixel butstepping the position of this pixel according to a coarser grid. Then,one can also detect very close inter-reflections which may otherwise behidden within a larger solid patch, but instead one risks missing smallreflection-causing features. By combination of single-pixel 71 and solidpatch 72 illumination as illustrated in FIG. 13 one can preferably beable to detect both.

By calibrating a coaxially mounted camera relative to the projector itis possible to transform any projected image into a primary-reflectioncamera image using “image rectification” functions (which are typicallyused in computer vision to speed up stereo matching by aligning thepixel rows from two cameras), or vice versa to transform a recordedimage to projector space. Thereby, lens distortion of both projector andcamera are taken into account as well as e.g. image shifts, rotationsetc. With a fixed set of patterns, this transformation can be done fromprojector to camera once for the full set of patterns, which laterreduces the processing time compared to transforming images on demand.

Although the invention is illustrated above, partly with reference tosome specific embodiments, it must be understood that numerousmodifications and combinations of different features of the embodimentscan be made and that the different features can be combined with eachother or with triangulation approaches known from prior art.

The invention claimed is:
 1. A method of identifying an object surfacepoint or region of particular measuring properties for opticalmeasurement of the respective point or region using an optical sensorwhich provides defined measuring conditions at least regarding emissionof measuring light (I_(D)) and reception of reflected measuring light(I_(R)) in a defined spatial relationship, the method comprising:defining a point or region of interest of the object; determining asurface property related to a visual characteristic of the defined pointor of at least a part of the defined region with respect to a particularoptical measurement using the optical sensor; and deriving an objectinformation, wherein the object information includes information ofmeasurability of the object with the defined measuring conditions basedon the surface property, the object information representing aninformation about an expected effect on the particular opticalmeasurement due to the surface property and measuring conditions,wherein: the point or region of interest is defined by use of a coaxialview to the object, the coaxial view includes a viewing axis of a camerabeing coaxial to an emission axis of the measuring light (I_(D)), anddetermination of the surface property is performed by: opticallypre-measuring the point or region using the optical sensor by:illuminating the point or at least a part of the region with themeasuring light (I_(D)) emitable by the optical sensor, capturing atleast one image by means of the optical sensor of at least oneillumination at the object caused by illuminating the object, andanalysing the at least one illumination regarding position or visualcharacteristic accuracy with respect to the measuring conditions of theoptical sensor.
 2. The method according to claim 1, wherein the opticalpre-measuring comprises: determining at least one image-position in theat least one image of respective illuminations at the object, checkingfor positional accuracy of the at least one image-position with respectto the measuring conditions of the optical sensor, and generatingposition accuracy information based on the checked positional accuracy.3. The method according to claim 2, wherein: generating image data ofthe at least one illumination, the image data comprising at least twopictorial representations of the at least one illumination at the objectfrom at least two different poses, determining the at least oneimage-position of the respective illuminations at the object for each ofthe pictorial representations, and checking the image-positions forconsistency regarding the measuring conditions.
 4. The method accordingto claim 3, wherein: checking if the image-positions represent a commonillumination based on an illumination direction for the measuring light(I_(D)), and comparing a spatial position derived by atriangulation-based determination based on the image-positions, with aposition of an illumination axis or illumination plane of the measuringlight (I_(D)).
 5. The method according to claim 1, wherein illuminationof the point or region is provided by the measuring light (I_(D)) beingin form of: a line of light, a light pattern, a light spot, or a patternwith spatially successive bright and dark illumination regions.
 6. Themethod according to claim 1, wherein the process of performing opticalpre-measuring comprises: moving the measuring light (I_(D)) over theobject according to a defined scanning path, continuously detecting aposition of an illumination caused by the moving measuring light,deriving a movement path for the illumination at the object, comparingthe scanning path to the derived movement path, and generating positionaccuracy information based on the comparison.
 7. The method according toclaim 1, wherein the optical pre-measuring comprises: analysing contrastor intensity of the at least one captured illumination, comparing thecontrast and/or intensity to a respective reference value, andgenerating visual characteristic accuracy information based on thecomparison.
 8. The method according to claim 1, wherein defining thepoint or region of interest comprises: defining a first polygon in afirst camera view of the object, defining a second polygon in a secondcamera view of the object, wherein the first and the second polygondefine a common region at the object, and deriving topographicinformation of the common region based on photogrammeric processingusing the first and the second camera view.
 9. The method according toclaim 1, wherein the optical measuring is performed as a pre-scanningprocess of the point or region.
 10. A non-transitory computer-readablemedium comprising a computer program product having computer-executableinstructions implemented for executing and controlling at least the stepof determination of the surface property of the method of claim
 1. 11. Atriangulation-based optical sensor comprising: a light emitting unitwith a light source for providing defined measuring light (I_(D))according to an emission axis; at least one light receiving unit havinga detector for detecting measuring light reflected and received from anobject to be measured; and a controlling and processing unit adapted toderive distance information based on the detected reflection, wherein atleast an arrangement of the light emitting unit and the light detectionunit with known spatial position and orientation relative to each otherdefines measuring conditions of the optical sensor, wherein thetriangulation-based optical sensor comprises a camera adapted to providereception of the reflected measuring light (I_(R)) according to aviewing axis, wherein the emission axis of the light emitting unit andthe viewing axis of the camera are coaxially aligned, and wherein thecontrolling and processing unit comprises a pre-measuring functionalityexecuting a determination of an object surface property related to avisual characteristic of a defined point or of at least a part of adefined region of interest of the object with respect to a particularoptical measurement using the optical sensor, the determination of theobject surface property being performed by: defining the point or regionof interest by use of a coaxial view to the object by means of thecamera and the light emitting unit, and optically pre-measuring thepoint or region of interest according to the following steps:illuminating the point or at least a part of the region with themeasuring light (I_(D)), capturing at least one image by means of thelight receiving unit of at least one illumination at the object causedby illuminating the object, and analysing the at least one illuminationregarding position or visual characteristic accuracy with respect to themeasuring conditions of the optical sensor, or analysing a digital modelof the object to be measured by performing the following steps:digitally aligning the digital model in accordance with an orientationof the object relative to the optical sensor, and determining visualcharacteristic properties of the point or region based on the alignedmodel regarding an illumination with the measuring light (I_(D)) in theorientation of the object relative to the optical sensor.
 12. Thetriangulation-based optical sensor according to claim 11, wherein: thelight emitting unit is embodied as a projector and defines an emissionaxis, the triangulation-based optical sensor comprises a camera whichdefines a viewing axis, and a projector object surface of the projectorand a camera image sensor of the camera which are arranged so that theemission axis and the viewing axis are coaxially aligned.